1. Field of the Invention
The present invention relates to an epoxy resin-free coating system based on thermoplastic polyesters. into which a polyester has been incorporated as an adhesion-promoting additive.
2. Description of the Related Art
On the basis of thermoplastic polyesters, coating materials have for a long time been produced and marketed for a very wide variety of applications. The common method of production entails incorporating the adjuvants required for the intended application, such as color pigments, fillers, stabilizers, leveling agents. luster agents and/or other additional and auxiliary substances, into the polyester on compounding extruders or kneading apparatus. The resulting compound can be applied as a solution, film, melt, powder or in some other form in the use envisaged.
A technique which is particularly suitable in practice is powder coating of metals by fluidized-bed sintering or by an electrostatic method. In the fluidized-bed sintering method, hot metal parts are coated by being dipped into an air-fluidized bed of the pulverulent coating material. In this procedure, thermoplastic melts onto the surface and, after cooling, forms a protective coating. In electrostatic coating, powder is sprayed by means of compressed air onto the metal part where it remains adherent through the application of high direct-current voltage. Subsequently, a coating is obtained likewise by melting in an oven or by using a gas flame, for example. The coating material can also be employed in another way, such as a melt or film, for example.
Despite the fact that the preparation of thermoplastic coating powders is relatively complex, due to use of a cold grinding operation with liquid nitrogen, it has been possible to maintain and even increase market share in the face of competition from wet coating materials and reactive systems. Environmental considerations are paramount since coating powders operate entirely without solvent. Additionally, purely thermoplastic coating materials have processing advantages over reactive systems since they are melted on purely through the supply of heat and cure by cooling. Because of this they are suitable, inter alia, for extremely rapid coating units operating with downstream further processing, for which reactive systems are generally too slow. By their very nature reactive systems require chemically reactive components and hardeners, which are often toxicologically objectionable. Moreover, a proportion of residual monomer remains in the coating after the reaction.
These are also the reasons why the use of thermoplastic systems is preferred in the weld seam protection of welded preserve cans. In the can, the can seam is reliably protected by thermoplastic polyesters against any ingredient, and hence preserves the contents unchanged.
The necessary coating properties, such as adhesion, elasticity and coating sterilizability, are not provided by the thermoplastic polyester alone. Marked improvements have only been obtained by the use of appropriate additives. Through such formulations properties have been optimized to the extent that coatings of thermoplastic polyesters can be attained having good service properties.
Adhesion promotion by the incorporation of epoxy resins as an additive into the thermoplastic polyesters is considered "state of the art". In this instance the epoxy resin is not--as is usually the case--chemically crosslinked but is merely mixed with the polyester, since the polyesters commonly employed do not possess any notable reactive groups. The activity of the epoxy resin additive in terms of the coating properties is excellent and therefore makes it possible to meet completely all requirements on the coating system.
When the coating system is employed in the food packaging sector, the relevant regulations must be met, such as FDA 175.300 (USA), Directive 90/128 (EU), etc. For such use, the thermoplastic systems, as already noted, have decisive advantages over other systems.
Recently, epoxy resins employed as a coating material in the food packaging sector have come under scrutiny. Traces of bisphenol A diglycidyl ether (BADGE) from epoxy resin coating materials leach out of the interior coatings of preserve cans and are taken up by humans upon consumption of the contents. In oil-containing fish cans, for example, impermissibly high concentrations have been measured of BADGE leach from the interior coating. BADGE is now suspected of having carcinogenic and estrogenic effects. Consequently, the Scientific Committee on Foods of the European Union in June 1996 undertook a toxicological reassessment of BADGE. Although there is no firm evidence of a carcinogenic effect to date, in November, 1996, the German Federal Institute for Consumer Health Protection and Veterinary Medicine called for reductions in food contamination. BADGE contents of more than 1 mg/kg are classified as objectionable on health grounds.
Customary thermoplastic coating systems include a small percentage of epoxy resin, in the formulation, which also includes a certain proportion of residual monomer (BADGE). Consequently, the thermoplastic polyester coating system, although fundamentally ecologically and toxicologically flawless, still provokes criticism due to the presence of epoxy resin therein.